The applicability of non-linear static analyses for the assessment of less-thanregular bridges has been subject of recent research by a number of authors ([11,13,14]), with mixed results. The study presented in the paper intends to provide a further example of application of the approach to a bridge whose characteristics are such as to test it in a rather extreme case. This is an existing important highway viaduct built in the sixties, of total length equal to 420m, with 11 spans of 33m each and a continuous RC deck pinned over the piers. These latter, whose height varies irregularly between 14m and 41m, consist of slender single-bay frames with intermediate transverse beams. Among the numerous proposals available, the multi-modal nonadaptive approach as proposed by Chopra et al. [4] has been chosen as the preferred compromise between accuracy and complexity. The selected structure is characterised by at least two modes providing comparable contributions to the transversal response. In view of the final comparison of the results with those from non-linear dynamic analyses, the critical question of the choice of the reference response dof to be monitored has been investigated as well as the approximation of the method as a function of the intensity of the seismic action (inducing a variable degree of inelasticity in the structure).

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